The present invention relates generally to the field of medical imaging, and more specifically to the field of tomosynthesis. In particular, the present invention relates to the visualization of reconstructed volumes from data acquired during tomosynthesis.
Tomosynthesis is an imaging modality that may be used in a medical context to allow physicians and radiologists to non-invasively obtain three-dimensional representations of selected organs or tissues of a patient. In tomosynthesis, projection radiographs, conventionally known as X-ray images, are acquired at different angles relative to the patient. Typically, a limited number of projection radiographs are acquired over a relatively small angular range. The projections comprising the radiographs generally reflect interactions between x-rays and the imaged object along the respective X-ray paths through the patient and, therefore, convey useful data regarding internal structures. From the acquired projection radiographs, a three-dimensional volumetric image representative of the imaged volume may be reconstructed.
The reconstructed volumetric image may be reviewed by a technologist or radiologist trained to generate a diagnosis or evaluation based on such data. In such a medical context, tomosynthesis may provide three-dimensional shape and location information of structures of interest as well as an increased conspicuity of the structures within the imaged volume. Typically, the structures within the reconstructed volumetric image, or within a slice, have a significantly higher contrast than in each of the respective projection images, i.e., radiographs.
However, evaluating the three-dimensional volumetric image may pose challenges in clinical practice. For example, viewing the volumetric image slice by slice may require viewing forty to sixty slices or more. Therefore, small structures present in a single slice may be easily missed. Moreover, the three-dimensional position and shape information, in particular the depth information (i.e., essentially in the direction of projection for the data acquisition), is only implicitly contained in the stack of slices, with the “depth” of a structure that is located within a given slice being derived from the position of that slice within the full slice sequence or the volumetric image.
To address these problems, three-dimensional volume visualization or volume rendering may be employed. These visualization techniques attempt to show the full three-dimensional volumetric image simultaneously, with the location and shape information being conveyed mainly through changes in view angle, i.e., perspective. In addition, volume visualization may be enhanced by including an occlusion effect, which hides (or partially hides) structures that are located behind other structures, depending on the view angle.
However, one drawback of many volume rendering methods is an associated loss of contrast, which may more than offset gains in contrast achieved by the three-dimensional reconstruction process. This problem typically occurs when showing the full volume from a view angle requires some type of averaging of values of the volumetric image for a range of depths. As a result, the perceived contrast of a small structure may be significantly smaller in the rendered image than in the original projection image data set. In addition, if a structure of interest is not located “close to the viewpoint,” i.e., in front of most other structures, as seen from the viewpoint, occlusion effects may further diminish the contrast of the structure, or even hide it completely. This problem may be addressed by visualizing, i.e., rendering, only the region or volume of interest within the volumetric image. This technique, however, requires either a priori knowledge of the volume of interest or an intelligent way of continuously adjusting the volume of interest during the volume rendering process to allow the visualization of any subvolume of the full reconstructed volumetric image. A technique for visualizing three-dimensional tomosynthesis data that provides good visualization of the three-dimensional context, i.e., localization and space information, without reducing contrast may, therefore, be desirable. Similarly, viewing modes which take advantage of the properties of such visualization techniques may also be desirable.